Information signals, such as data signals, media signals and especially compressed video and audio streams and more especially packetized audio and media streams propagate over various communication channels, such as but not limited to terrestrial, wireless, satellite, wireline and cable communication channels. Media streams usually include large amounts of information.
Digital transmission and compression techniques allow for transmitting media signals over communication channels in a compressed form. The Moving Pictures Experts Group (MPEG) specifications are standardized methods for compressing and transmitting media signals such as video and audio.
Communication channels that are connected to clients have a limited bandwidth, said limitation is known in the art as the “last mile problem”. Accordingly, only a certain amount of services can be provided to clients over said limited bandwidth channels. The amount of services is especially limited when some of them are bandwidth consuming.
It is noted that certain bandwidth limited networks are known in the art as broadband networks, but the term “broadband networks” mostly emphasizes the much larger bandwidth these communication networks have (usually about 750 Mhz) in comparison to even more bandwidth limited communication networks and technologies, such as Plain Old Telephone Networks, dial-in modems and the like.
Clients are grouped in service groups. A service group includes a plurality of clients, such as set top boxes, cable modems and the like, that receive the same multiplexed downstream signal (‘downstream’ means from service provides to a client, while ‘upstream’ means information transmitted from the clients) as they share the same communication link/output port of a node, hub or even a Headend. A service group is also known in the art as a “forward carrier path”.
Typically, media packets are provided to Headends, primary hubs, secondary hubs and nodes and then are transmitted over communication networks to the clients. Some media packets can pass through more than one entity out of said Headend, hubs and nodes. In order to transmit the media packets over said networks they are modulated in various manners known in the art.
The modulation is implemented by modulators that may be located within said entities. The cost of modulators, especially modulators that are used to broadcast packets to a very large amount of clients, is high. Furthermore, many modulators can be involved in modulating the same media packets to multiple clients, thus reducing the overall amount of services that is provided to clients.
A typical modulator is a Quadrature Amplitude Modulation (QAM) modulator, although other modulators, such as QPSK modulators, are known in the art. QPSK modulation is more robust than QAM modulation but is slower. It is noted that QPSK modulation may be used for modulating upstream information.
A QAM modulator usually receives input signals and outputs a modulated signal over a Radio Frequency carrier. Usually said modulator is also able to perform additional processing steps such as encryption, error correction coding, interleaving and the like. Most QAM modulators are able to alter the carrier frequency within a predefined frequency range. The modulators are arranged in arrays that are also known in the art as multi-modulators. Multi-modulators provide multiple modulated output signals. Modulated signals that are spaced apart in the frequency domain can be combined. Typically, each modulator outputs an output channel that has a bandwidth of 6 Mhz. The 6 Mhz channel can be used to convey a single analog television channel or ten MPEG compliant television channels. Typically, downstream channels are in the 50-750 Mhz band while upstream channels are in the 5-40 Mhz band.
U.S. patent application number US 2002/0162114 of Bisher JR et al. describes a system and method for multicasting packets in a subscriber network. The method is able to determine whether a media packet is a unicast packet or a multicast packet. In response to the determination the media packet is sent to a unicast buffer and a unicast modulator or is sent to multicast buffers and multicast modulators.